The measurement of fluid velocity is important in many industrial applications such as for instance, the measurement of the speed of coolant flow through pipes, as well as in scientific and medical applications, for instance in the measurement of fluid velocity in the wakes of moving vessels or models of vessels. Provided that the fluid carries scatterers capable of reflecting ultrasound such as tiny air bubbles in the case of water or liquid starch suspensions or red blood cells in the case of blood, pulsed doppler ultrasound systems have been found to be advantageous over other means of measuring fluid velocity. In the known doppler ultrasound systems a short burst or pulse of radio frequency sound is emitted from a transducer and sent towards the moving scatterers. The reflected echo is shifted in frequency because of the doppler effect, and by processing this echo in a receiver and measuring the shifted frequency with respect to the transmitted signal it is possible to estimate the velocity of the moving scatterers and thus of the fluid that contains them. Present techniques of measuring fluid velocity around ship models for instance, involve placing detectors at various points where the velocity of the liquid is to be measured. Using a pulsed ultrasonic doppler system, if it were not for the range limitations described below, it would be possible simply by pointing the ultrasound beam in different directions and adjusting the range, to obtain the velocity at any point in a fluid with only a single instrument, and to obtain the velocity over a whole region of fluid by rapidly scanning over the region at electronic speeds.
In medical applications, pulsed doppler ultrasound systems are used at this time to measure the blood velocity in peripheral blood vessels, i.e., vessels that lie close to the skin, with the transducer being positioned outside the body. This is a much more pleasant and convenient way of measuring blood velocity than other known techniques which involve cutting into the skin. It would be convenient and advantageous if it were possible to use this known technique to measure the blood velocity in vessels which are deeper in the body such as those near the heart, but this is at present impossible because of the range limitations of pulsed doppler systems.